7: Cardiac Conduction And APs Flashcards
Spread of AP through heart
SA node -> AV node -> Purkinje fibers -> Bundle of His -> R and L bundle branch -> L further divides into smaller Purkinje fibers
Which side does the AP go through first, right or left?
right - in atria and ventricles
Which layer of the heart receives the AP first
Endocardium (before epicardium)
What parts of the heart can have pacemaker qualities if the conditions are right?
- SA node
- AV nodE
- Purkinje fibers
- Bundle of His
- RBB and LBB
What quality of a heart cell allows higher velocity conduction of AP? Which cell types are fastest vs slowest?
Larger diameter - Purkinje > myocytes > AV node
Where do fast vs slow cardiac APs occur?
Fast: myocytes, Purkinje fibers
Slow: SA and AV nodes
What do INa and Ito stand for?
INa: Na conductance
Ito: outward K current
What causes cells undergoing slow APs to act as pacemakers?
Slow influx of Na through funny voltage-gated Na channels that open upon complete repol of membrane
Chronotropic
Affects HR
Dromotropic
Affects conduction speed
Inotropic
Affects contractility
Refractory period
When AP cannot be generated / is more difficult
Why are refractory periods longer in the heart? Two reasons
- Aid in rhythmic contraction
2. Protect against arrhthmias
Two types of refractory periods in the heart
- Absolute (ARP): No AP can be generated
2. Relative (RRP): AP requires much greater stimulus and/or will have abnormal conduction
What causes absolute vs relative refractory periods?
Absolute: all voltage gated Na channels are inactivated
Relative: many voltage gated Na channels have reset, but not all
Supranormal period (SNP)
Cell more excitable than normal, but AP will be abnormal
What two things occur in the phases of the cardiac AP due to hyperkalemia?
Slows phase 0 + speeds up phase 3 -> messes up refractory period and shrinks the AP
Interval vs segment in ECGs
Interval: includes entires waves involved
Segment: starts at end of a wave -> ends at beginning of a wave
What interval on ECG is used to determine HR
RR interval
Bradycardia vs tachycardia numbers
Brady: <60 bpm
Tachy: >100 bpm
PR interval of a first degree AV block
PR interval > 0.2 ms
Long QT syndrome - what is different in the AP phases?
Transition from phase 2 to 3 is lengthened
Determining HR on ECG: time between dark vs light lines
Dark lines: 0.2 seconds
Light lines: 0.04 seconds
